Over the course of recent years, wireless communication networks and mobile radio networks have developed to become an important component in the communications infrastructure on a worldwide scale. As a result of the enormous demand for mobile radio capacity, various industry-standard communication technologies have been developed based on digital modulation techniques. Thus, for example, Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) and frequency hopping techniques have been used in order to develop modern communication systems. Following parallel implementation of systems of these types, it has proven to be advantageous to provide receivers which are capable of operating with one or more of these standard techniques. In order to make this possible, it is necessary to provide a receiver which is capable of receiving signals that have been modulated using different modulation techniques.
In Central Europe, in particular, but also in other regions of the world, the GSM mobile radio system has been developed in this context, whereby different frequency ranges are made available within this system. The next few years will see the advent of the next generation of mobile radio standard (UMTS, 3G) which will encompass additional frequency ranges.
From the standpoint of the manufacturer of terminal equipment, it is, therefore, becoming increasingly more difficult to guarantee signal generation for the different bands and standards at an acceptable level of resource deployment and, conversely, to provide the corresponding receive facilities; in particular, on the condition that the number of oscillators used and their required tuning range should be kept as small as possible.
Known receivers for processing signals employing different modulation standards implement, for example, the double conversion receiver architecture. The double conversion receiver architecture is distinguished by the fact that the received radio signal is converted to an intermediate frequency signal and the intermediate frequency signal is then converted downward into the base band. In addition, the intermediate frequency is also delivered to a gain check. However, double conversion receivers have the disadvantage that they require a large number of circuitry components, which increases the costs, size and power draw of the receiver.
An alternative to these double conversion receivers is offered by direct conversion receivers which have no intermediate frequency and, therefore, have no resource requirement with respect to filtering for the intermediate frequency. For receiving, two-thirds or four-thirds of the oscillator frequency, which is used for downconversion of the receive signal into the base band, is optionally generated from a first local oscillator or from a second local oscillator via a regenerative divider.
In this situation, for transmission, use is made of the fact that the regenerative divider also generates one third of the oscillator frequency as a “waste product”. This is subsequently IQ-modulated and delivered to an offset PLL (phase-locked-loop) which steps it up to the actual transmit frequency. For the special UTMS application situation, the output signal is generated by mixing with the first local oscillator since, in this case, an amplitude-modulated signal must be present, which cannot be achieved with an offset PLL.
The disadvantage of this solution is the fact that the first local oscillator must have a broad tuning range from 1282.5 to 1452.5 MHz. Not only is this difficult to implement but, under certain circumstances, it even necessitates the use of two separate oscillators for the respective sub-ranges. Furthermore, the offset PLL likewise contains two oscillators internally which make significant demands in terms of their space requirement.
An object of the present invention is, therefore, to provide a method and an apparatus of the generic type in which as small a number of oscillators as possible is used and the requisite tuning range is as small as possible. As such, the solution should be capable of being implemented as cost-effectively and simply as possible.